Interpretive Summary: A genetic system for the study of pathogenesis of Lyme disease has yet to be developed. While investigating methods for creating mutants in Borrelia burgdorferi, the causative agent of Lyme disease, erythromycin resistant isolates were obtained. Resistance did not appear to be due to intrinsic factors, inactivation of the antibiotic, or an efflux pump, but did appear to be due to the presence of an erythromycin resistance methylase (erm). In addition, the resistance could be transferred from B. burgdorferi to both Enterococcus and Bacillus. Because erythromycin was a drug used to treat Lyme disease, this information will be useful for clinicians who prescribe the drugs for treatment. This information will also be useful for researchers who can now use erythromycin as a genetic marker in the bacterium.

Technical Abstract:
We report the identification of isolates of Borrelia burgdorferi strain B31 that exhibit an unusual ML or MLSA antibiotic resistance pattern. Low-passage (LP) isolates were resistant to high levels (>100 ?g/ml) of erythromycin, a 16-membered ring macrolide, spiramycin, and the lincosamides (lincomycin, clindamycin) but sensitive to dalfopristin, a streptogramin type B antibiotic. Interestingly, the high-passage (HP) erythromycin resistant (eryR) strain B31 was resistant to quinupristin, a streptogramin type A antibiotic (25 mg/ml). Biochemical analysis revealed that eryR was not due to antibiotic inactivation or energy-dependent efflux but instead to modification of ribosomes in these strains. Although a mutation was detected in the RspL4 gene in one eryR strain, this mutation was not responsible for the ery resistance phenotype, suggesting that the most likely mechanism for eryR in these strains is modification of the 23S rRNA. Most interestingly, using bacterial matings, we were able to demonstrate high frequency transfer of the eryR phenotype via conjugation from B. burgdorferi to Bacillus subtilis (10-2 to 10-4) or Enterococcus faecalis (10-5). Transfer was dependent on cell to cell contact, was reduced in absence of RecA in the recipient, and a resident plasmid, pAD1, in E. faecalis, eliminated transfer. An interspecies conjugal system in B. burgdorferi suggests that horizontal gene transfer may play a role its evolution and is a potential tool for developing new genetic systems to study the pathogenesis of Lyme disease.